Multi-layer films and sheets are widely used for food packaging applications. Depending on the intended application, the number and arrangement of resin layers and the type of resins employed will vary. Polyethylene resins are often included as one of the layers for food contact and sealing properties. Ethylene-vinyl alcohol (EVOH) copolymers and polyamides (nylons) are widely used as oxygen and moisture barrier layers. Styrenic polymers are commonly included as structural layers, especially for thermoforming applications. Styrenic polymers which contain rubber, such as high impact polystyrene (HIPS), are especially useful for this purpose. However, adhering dissimilar resin layers in multilayer films and sheets is always challenging. While numerous tie-layer adhesive compositions containing modified polyolefins are effective in adhering polyolefin resin layer and barrier resin layers, they often lack adequate adhesion to styrenic resin layers.
U.S. Pat. No. 5,709,953 discloses an extrudable adhesive composition for bonding polystyrene resin to a second resin such as oxygen- or nitrogen-containing resins, such as EVOH, polyesters, and polyamides (nylon) or to non-polar resins, such as polyethylene. The extrudable adhesive comprises an ethylene polymer and styrene/aliphatic/styrene block copolymer. The reference discloses that all or a portion of the ethylene polymer fraction can be grafted with an unsaturated carboxylic acid or anhydride. Similarly, U.S. Pat. No. 6,184,298 discloses an adhesive composition which consists essentially of a polyethylene polymer and an unmodified styrene-based elastomer having an immeasurable Mooney viscosity.
There is a continuing need for adhesive compositions which provide improved adhesion to styrenic polymer layers in coextruded multi-layer films and sheets.